Separation and purification of isoprene



SePf- '24, 1946- I J. A. PA1rERsoN 2,407,997

SEPARATION AND PURIFICATION oFiIsoPRENE' 3E.it eww/ "SEPARATION AND PURIFICATION oF IsoPRENE J, A. PATTERSON Filed. Aug. l'7 1943 2 sheets-sheet` 2 PGY- L4 Buff,

Patented Sept. 24, 1945 SEPARATION AND PURIFICATION F ISOPRENE John A. Patterson, Westfield, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application August 17, 1943, Serial No. 498,944

2 Claims.

This invention relates to the treatment of mixtures of hydrocarbons of the C3 to Ca range to effect segregation and purification of paraffins, mono-olens and dioleflns. The invention relates particularly to the separation of diolens containing carbon atoms in the molecule from their admixture with other hydrocarbons in products of the treatment of petroleum fractions; and also the purification of diolens.

The extraction of individual C4 and C5 hydrocarbons from easily available sources is beset with many difficulties because of the low concentrations involved and the interrelated properties of these hydrocarbons in such supplies. Thus, the hydrocarbons containing 5 carbon atoms in the molecule have boiling points at 760 mm. and relative volatilities, a as follows:

-methyl butene-i Pentene-l Cis-piperylene Cyclopentene. Cyclopentane The boiling points and the relative volatilities of the various C5 compounds clearly do not follow the degree of chemical unsaturation of the compounds.

a complex mixture would also clearly require extremely expensive equipment. Moreover, if a fractional distillation were to be made upon a general mixture of C5 hydrocarbons to separate close boiling fractions, prolonged heat under high reflux ratios would be required and under ,such conditions considerable polymerization of the more unsaturated hydrocarbons would occur. Thus, fractional distillation alone as a means of making a separation of hydrocarbon from complex mixtures is disadvantageous, even in the laboratory. y

VIn a co-pending application, Serial No. 470,426, filed December 29, 1942, it was disclosed that complex hydrocarbon mixtures could be advantageously resolved into their individual constituents by a unique combination of at least three processing steps. In two of the three steps fractional distillation is employed. Between the two fractional distillation steps is eiiected an extractive distillation. The process of that invention is thus the combination of a carefully controlled fractional distillation of Va suitable feed stock followed by an extractive distillation rof a particular distillate fraction and then subsequently Even to effect separation upon the basis of normal vapor pressure, of a simple from fractionally distilling one of the particular distillate fractions from the extractive distillation operation. Invention was stated to reside not only in the combination of the fractional distillation steps with extractive distillation, but also in the particular manner of combination in order to achieve particularly desirable fractions for the second and third steps in the processing, namely, for the extractive distillation and the second fractional distillation steps. It was stated also in Serial No. 470,426 that in commercial operation t was usual to employ a fourth step, namely, the recovery of the solvent employed in the extractive distillation step. Such a step is, however, incidental rather than an essential feature of the process of the invention.

Thus, according tothe process disclosed in application Serial No. 470,426, the concentration of C5 dioleiins was eifected by supplying the system with a feed stock consisting of a narrow C5 cut as rich as possible in the desired diolen, isoprene `for example. A debutanized aromatic distillate containing 4% of isoprene was fractionally distilled in a 50 plate tower with a reflux ratio of from 5 to l0, depending on the nature of the feed stock, and a product was had containing from 15% to 25% of isoprene. Such fractional distillation failed to remove some of the hydrocarbons most difcult to separate in the subsequent extractive distillation. In the extractive distillation with aqueous acetone as the solvent, trimethylethylene which has a normal volatility relative to isoprene of 0.88, has a volatility index a relative to isoprene of 1.23. Thus in both the fractional and extractive distillation systems isoprene and trimethylethylene have about the same spread. The present invention overcomes this deficiency.

It has nowbeen found that by substituting in the processing disclosed in application Serial No. 470,426, an azeotropic distillation step for the rst fractional distillation step, concentration of the desired dioleins may be easily effected and distinction also made between hydrocarbons otherwise diicult to separate by fractional and extractive distillations either alone or in combination. By adding a controlled quantity of a third liquid or solvent to the feed fractionating tower so that the distillate will consist of azeotropic mixtures, separation can easily be effected of many different hydrocarbons. Thus, in mixtures containing isoprene and trimethylethylene in association with other C4 and Cs hydrocarbons and employing aqueous acetone as the solvent, there is formed a ternary azeotrope of isoprene, acetone and water of lower boiling point than that of any other Cs hydrocarbon usually present in such a mixture with the exception of pentene-l which is otherwise easy to separate from isoprene. The volatility of trimethylethylene re1- 3 ative to isoprene is depressed, making otherwise difficult separation: relatively easy. Since pen tene-1 has a norma1 volatility greater than that of isoprene, the separation of this and lighter hym drocarbons is easily effected in the subsequent extractive distillation step disclosed in application Serial No. 470,426. The following table gives the average relative volatilities of the various compounds in azeotropic distillation:

Relative volatility at 25 lbs/sq. in. gauge Component Normal Azeotropic Extractivc Y distillation distillation distillation Pentene-l 1.06 1. 12 1. 36 Isoprene 1.00 1.00 l. Trimethylethylene. 88 78 l; 23'

In order to furnish a better basis for appreciating the invention, analysis of the processing inV each of the essential steps is presented; In fractional' distillation, separation of the constituentsof a mixture is made upon the basis of differences at varying temperatures of the vapor pressures of ious compounds are greater in the-presence" of the? 1n the presence andabsence of aqueous acetone:

` With 2' No .volumes of solvent aqueous acetone Isoprene 1. 00 1.00 n-Pentane 0.92 1.43 Pentene-2 (trans) 0. 92 1.30 Trimethyl'ethylene. 0. 88 1. 26 Cyclopentene 0. 7l 0. Q5

It is. such an elect. under suitable conditions of pressure. and concentration of the added liquid that. contributes to the formation of azeotropic compositions.

The added liquid inA extractiva distillation is employed in greater amounts than in azeotropic distillation in order that the maximum effect of the4 solvent may be exercised upon the diierent groups of. compounds; that is, the amount of ex-v traneous. liquid employed in an extractive distillation isz greatly in excess of that which would form azeotropic mixtures in the system. Thus, the addition vof an extraneous liquid in extractive distillation is related to, but very distinct from, the addition of a denite amount of the added'v liquid in an azeotropic distillation system.

In extractive distillation andv Liquids suitable in extractive distillation are in general the stable organic liquids of high dielectric constant. The suitable materials are therefore the normally liquid polar organic compounds containing oxygen, nitrogen, sulfur and related elements. Suitable liquids also have the following characteristics:

(l) High solubility for the hydrocarbons;

(2) Marked effect on the relative volatility;

(3') R'eadily separable from the hydrocarbons by distillation 0r when azeotropes are formed by solvent extraction or other means;

(4) Sufficient volatility to permit stripping with..

out requiring the use of high temperatures;

5). Comparative stability when heated in the presence of hydrocarbons; and

(65 Commercial availability andr non-corrosiveness.

In the case of the" separation of theC hydrocarbonsA from their admixture withvother hydrocarbons, acetone, furfural, pyridine, the amines and their admixtures with waterY have been found' to be particularly advantageous.

In the case of azeotropic distillation' the same type of liquids are suitable as solvents. In addition, however, the liquids should havethe following characteristics:

(l)V Ability to form minimumboiling azeotropes withthe desired hydrocarbons;`

(2) `lteadily separable from the hydrocarbons;

(3) Suicient diiierence in volatility between the solvent and the azeotropes;

(e)Y Comparative stability when heated inA the presence of hydrocarbons; and

5) Commercial availability and noncorrosiveness.

Suitableliquids therefore for azeotropic distillation are the saturated ketones, the amines, the alcohols and their admixture with water. l

It will. thus be noted that liquids advantageous as` solvents in both extractive and azeotropic dis-- tillation systems have very closely related proporties and that, in general, liquids suitable for one type of distillationl are alsov suitable for the other type or distillation. The employment of azeotropic mixtures as a means of4 extractionof the desired hydrocarbonsprovides the basis for the separation of' compounds otherwisedifcult, and advantages in'over-all proeessingyespecially when the same liquid is employed. in the azeotropic.. and extractive distillation systems. The.` particular.v processing advantages will be; evident from thev further description and illustration. of theinvention.

Taking the separation of isoprene as a suitable example of the process of this invention, the first step involves the azeotropic distillation of anarfrow C5 hydrocarbon fraction containing a sub*- stantial amount of isoprene and rejecting as much as possible of the piperylene, cyclopentadiene, trimethylethylene and the pentenes.V A suitable source for isoprene is the refinery Icy-product distillate from the high temperature cracking of gas' oil, Virgin' naphtha, kerosene and/or othersuitable feed stocks. A typical debutanized stock from'thisV source may contain about 4% isoprene.

A stock of this nature is distilled in the presence. of an entrainer so as to separate asa dis-l tillate, essentially all ofthe desired hydrocarbons as constituents ofazeotropic mixtures and as residue, undesirable hydrocarbons of C5 content and of higher boiling points. Aqueous acetone is usually employed as the entrainer in the separation of C5 hydrocarbons containing a high percentage of isoprene but any of the lower saturated ke-` The distillate material is then subjected to an' extractive distillation in the presenceof an added amount of a solvent liquid which alects in different degrees the relative volatilities of the various type hydrocarbons. For this purpose usually the same liquid as in the azeotropic distillation is employed sincethe liquid is largely selected on this basis; 'for example, aqueous acetone in the extraction of isoprene from close-cut C5 hydrocarbon fractions. Other organic liquids such as acetone itself, furfural, the liquid basic nitrogen compounds such as pyridine and the amines and mixtures thereof with water, may also be advantageously used. In the case of aqueous acetone as the added liquid, the total amount em` ployed is usually about twice the volume of the liquid hydrocarbon mixture owing down through l the distillation tower: in the case of furfural, the ratio is as high as 7 to 1.

When diierent added liquids are used in the azeotropic and extractive distillation operations, separation of the added liquid from the azeotropic distillate is necessary prior to subjecting the distillate product to extractive distillation. This may be accomplished by extraction with water or other known means. The use of different added liquids has the disadvantage of necessitating separate recovery systems for each liquid. The gain in product purity and saving in heat expenditure make this method of operation economically advantageous in some cases.

Thus, under suitably controlled conditions in the presence of such quantities of added liquid, such as aqueous acetone, 'a vapor overhead product is obtained from the extractive `distillation system which consists essentially of paraffins and monoolens of C5 molecular content. The residue which is usually obtained consists essentially of solvent which is recycled to the proper point in the tower to maintain as high a solvent to hydrocarbon ratio as is advantageous for the process. The most important fraction removed from the system is the intermediate distillate material conveniently termed the side-stream product. This side-stream product consists mainly of isoprene with a lesser amount of piperylene and such traces of liquid polymer products as were formed during the distillation operations. Both the overhead and side-stream distillate fractions may contain substantial quantities of the added liquid-that is, aqueous acetone in this case-as a result of the formation of azeotropic mixtures.

The side-stream product from the extractive distillation operation is normally treated to effect the separation of the liquid added in the azeotropic and extractive distillation operations. When aqueous acetone is employed as the added liquid, the side-stream product is usually countercurrently treated with water to remove the acetone, and thus to permit the isoprene and other hydrocarbons to be separated therefrom. 'The hydrocarbon material after the separation of the added liquid is then fractionally distilled under carefully controlledconditions to obtain as an overhead product one of the hydrocarbons in substantial purity. In the present case the fractional distillation is eiected to obtain substantially` pure isoprene as the overhead product; and to leave as distillation residue the piperylenes, and cyclopentadiene not previously removed due to the presence in the system of pentanes, and smaller quantities of other impurities. l

Since some polymerization may `occur durin the azeotropic and extractive distillation steps, high-boiling polymeric materials are normally present in the solvent recycle. Inlorder to mainl tain the polymer content of the solvent at a low value, usually about 3% to 5% of the recycle solvent stock is removed and combined with the,

overhead distillate obtained from the extractive distillation; and the combined stream is then treated for solvent recovery.

In order to illustrate the invention more clearly, the following detailed description of processing is presented. The reference numerals given in the description refer to \the accompanying drawings which present a `suitable lay-out `of equipment, and indicates a suitable flow of materials. for processing according to the invention. In this illustration, example will be taken of the separation and purication of isoprene as obtained from the treatment of a debutanizeddistillate stream from a liquid phase cracking of a virgin gas oil stock, A specific example of a typical debutanized distillate stream has the following molar percentage composition:

Butenes 3-"nethyl butene-l 0.5

Pentene-l 9.4 2-methyl butene-l 2.0 Isoprene 4.1 Trans pentene-2 1.0A N-pentane 0.8 Cis pentene-Z` 1.0 2-methyl-butene-2 2.2 Cyclopentadiene 1.3 Y Trans piperylene l2 9 Cis piperylene l Cyclopentene 2.1 Cyclopentane 0.1 C6 11.7 Cs aromatics 11.7 C7 18.7 C8 9.8 C9 20.2

The feed stock is supplied to fractionating equipment l0 (Figure l) through line Il. The fractionating equipment I0 may be any of the usual types, such as a fracticnating tower containing bubble plates. The equipment is usually operated at about atmospheric or somewhat above atmospheric pressure. In the diagram, equipment l0 is specifically shown as being a tower of multi-plate construction containing bubble cap plates. In normal operation of processing according to the invention, about y50 plates are present insuch a tower. The tower is shown as being complete with an overhead vapor line I2, a condenser i3, a reflux drum 14, a reflux line l5, a bottoms line i6, a reboiler I8, reboiler vapor line IS, and an entrainer supply line ISA. The

distillate product is withdrawn from the distillation system through line 2l` and the bottoms product from the system through line l'l. The amount of entrainer liquid supplied to the system 4through line 15A is dependent upon the content of isoprene in the feed stock so as to form therewith theazeotropicY composition but insuffi-` cientv in` amount tol form with anv azeotropic composition. with trimethylethylene that is in.

this caseabout one volume of aqueousfacetoneto ten volumes of the: feed stock.,

The pressure upon* the. system for. the treat,- ment of the specific composition is maintainedabout 25 lbs. per sq; in. (gauge), in order" to be able to employ ordinary water for cooling purposes in the condenser I3. The degree of separation in tower I is controlledby theheatsupply fromthe reboiler I8.' andthe ratio of reiiuvx supplied through line I'. 'Iheipercentage ofthe feed takenoff as product through line.. 2|. is? adjusted so as to take thefmajor. portion of the isoprene. in this fraction. while rejecting' almost all of the trimethylethylene: and. most; of the piperylene and pentenesv in the residue; In the example used for illustration,al reflux ratio of'5. is maintained whiletaking off approximately 6% to 7% of the feed as distillate product. Under these conditions, the temperature at the top of the tower is 140 F., while the temperature at the exit of the line I'I is about 275 F.

The overhead product removed through line I2 contains almost all the aqueous acetone added through line IEA together with the majority of the isoprene and some of' the piperylene in the mixture, some of the pentene-l and traces of other C5 olefins and parafns as azeotropic mixtures' formed between the constituents of the original hydrocarbon mixture.` In this distillate the azeotrope containing. isoprene contains 92.0% isoprene, 7.6% acetone and 0.4% water. Isoprene is thus distilled at a temperature 3 to 4 F. below its own boiling temperature thereby minimizing heat eects productive of polymer formation. Moreover, as a resultof the high concentration of the isoprene inthe distillate material, a smaller quantity of distillate is taken off as compared to straight fractional distillation, thus effecting considerable heat and processing economies. The heat` economy involved more than compensates for the heat input required 'for distillation of theY added aqueous acetone.

The entire distillate is condensed in equipment I3 and passed to the drum I4. From the drum I4, a quantity'of reflux is passed through line I5 in order to maintain the desired operating conditions. The remainder of the condensed distillate is withdrawn from the drum of the redux to product is between 4 and 5, depending upon the feed. In the processing of the vfeed stock of the specific illustration, the composition of the distillate material under. the specific operating conditions given shows the following molar percentagecomposition:

I The residual material containing the tri-methyl'- |4 and passedv through line 2| for furtherprocessing'. The ratio rco 8i: ethylene, the: piperyleneV normalpentanav some pentenes, and someisoprene-in addition tohigher boiling.l hydrocarbons,A is withdrawn through line I6.v A portionv4 oi the residue is passed through the reboiler I8 where it is vaporized in order to' supply the=heat necessary, to operate thetower, the vapor being returned; to-` the tower I0 through line I9?. The remainder of the higher boiling material is'removed from the system through 1ine.- I'I.

The overhead fraction fromtower I0 as re'- moved through-line 2| is pass'edltothe fractionating tower-20; Thetower 20 issimilar in design to tower Ill and maybe any type of;- suitable' fractionating equipment. The tower 20', however,. is. of' smaller dimensionsg than the corresponding tower. employed when fractional distillation is. used in the preparation of thefeed material such as inthe processing disclosed inthe ,copending applicatiomserial No'. 470,426. TheY reduced` size of the' tower 20 is occasioned by the smaller amount of the v distillate and the higherl concentration therein of the desired hydrocarbons in the feed supply to the extractive distillation system'than' when fractional distillation: isfemployed inthe tower I0. vapor line 22, a condenser 23; a reflux drum24, a reflux li-ne 25,. a bottomsl line 2G,- a reboi-l'er5 28 andI a reboiler vapor return line- 29. The tower also has a-solvent line- 33, a side streamvapor line and aside stream return-line 35. rEhefdistillate product is withdrawn from: the system through line 4I and the bottoms product through line 2l. The pressure upon'the system is maintained at about 25-lbsz per sq. in. (gauge), in order to have aworkab'le condensing temperature similar to that prevailing inthe fractionatingequipment I0.

The yoverhead distillate'vapor removed through line 22 consists of. azeotropic mixtures'of" C5 oleiins and parains when aqueous acetone is employed as the` solvent material admittedth-rough line 39- The'vapors are'condensed in equipment 23 and passed tothetreux drum-1.24.V A portion of the condensatexisreturned; through linev 25 as reux, while the remainder is withdrawn through line 4|-, cooledl in equipment 42 y(Figure 1A) and conductedthrough linet3-to the water scrubbing equipment 5I).

The quantity of aqueous acetone admitted through line 39 l to the. extractive distillation sys'- tem isfforthevamount and composition of feed supply in a volumeratio of solvent to azeotropic mixtu-re Vof 2--1y from.u the v point of` introduction tothe plate upon` which. thefside stream isV with-` drawn.` Belowthe plate. from which. thev side" stream prod-uct iswithdrawn, the solventl is stripped of its hydrocarbon: content.

The bottomsw-ithdrawnthroughh line 26I consist of relatively pure solvent. bottoms liquid'is passed through the' reboiler 28 where itis vaporized; the vapors being returned to-the tower 20. through the Vvapor line 29, to' supe ply the heatnecessary to operate the tower. The remainder of the solvent, except for av small portion withdrawn for purification, as to befurther described,.is conducted through line 21 tocooler 38 and then through line 39v to the upper portion of the tower 20. Thus, the'major part of the solvent continuously recycles within the extractive distillation system.

Tlevapor sidev stream removed through line 3| contains a high concentration of` i'soprene with small" amounts. of' otherv diolens in the presence of` solvent in anamount ineequlibrium with a 75 molar' percent" concentration of solvent on th'e It is furnished withanoverhead A portion of the' 9 plates of the extractive distillation tower 20. The volume of the solvent is about 50% of the vapor mixture. This side stream material withdrawn through line 3| is conducted to the base of the side stream concentrating tower 3|). The tower 3) may be any suitable fractionating apparatus, such as a tower provided with bubble plates, as previously employed. The tower is provided with an overhead vapor line 32, a condenser 33, a reflux drum 34, a reflux line 35 and a bottoms line 36, in addition to the vapor inlet line 3|. The vapor entering the tower 3|] through line 3| supplies the heat necessary to operate the tower. The overhead distillate removed through line 32 consists essentially of the azeotropic mixture of isoprene, acetone and water and contains in addition small quantities of other C5 unsaturates and higher boiling materials such as polymers formed during the extractive distillation operation. The vapor is condensed in equipment 33 and conducted to the drum 34. A portion of the condensate is returned to the tower 30 through line 35 as reflux, while the remainder is withdrawn through line` 3l to the water scrubbing equipment 4i). i

The degree of separation of the hydrocarbons taking place in the extractive distillation system 2B is dependent on two functions, namely, the ratio of the solvent to hydrocarbon liquid on the plates and the ratio of liquid to vapor iowing. through the tower. The `ratio of solvent to hydrocarbon is controlled by the rate of solvent recirculation. The ratio of liquid to vapor in the zone ofA high solvent to hydrocarbon ratio, that is, between the solvent inlet and the side stream outlet, is controlled by the reflux returned through line 25 from the reflux drum 2H. Due to considerable difference in solvent concentration between the top of the tower and the extractive distillation zone and the difference in latent heats` of the constituents,'thereflux ratio andfconse- Vquently the ratio of liquid to vapor `at the top of the tower must be appreciably higher than the liquid to vapor Vratio in the extractive distillation zone.

In the case of the specific example, the liquid to `vapor ratio in the extractive distillation zone is 0.80, while the ratio at the top of the -tower is 0.85, the latter being equivalent to a reflux ratio lof 5.5. The reux ratio used in the side stream, tower 3|! is adjusted so asto obtain an overhead product consisting essentially of the azeotropic mixture of isoprene, acetone andwater and to supply adequate reflux to the solvent stripping section of the tower 2i); whichever requires the,...

supplied completely from the single` reboiler '28.,`

In this manner, considerable heat economy is effected, since the heat to operate the extractive distillation zone in tower and the diolen concentrating side stream tower 30 are supplied by condensation of the solvent mixture at the top of the solvent stripping section of the tower. If this were not done, a condenser would have to be provided for the solvent stripper to supply the .reiiux necessary for operation, and additional heat would be required to operate the extractive `10 distillation system and to effect the side stream concentration. In the same way, the condensation needed to produce the required reux in the extractive distillation zone is obtained by supplying the heat needed toconcentrate the monoolens at the top of the tower 20.

In the operation of the system comprising towers 20 and 33 and their auxiliaries, separation into the zones indicated above is clearly dened by the various temperature Zones. Thus, the vapor leaving the top of the tower 20 is at 140 F. The temperature rises sharply to 155 F. between the topl of the tower and the plate on which the solvent is admitted through line 39, then very slowly rising to 160 F. at the plate where the side streamis withdrawn through line 3|. Below the plateirom which the side stream is removed and as a result of the concentration of the aqueous acetone, the temperature rises sharply to 208 F. the temperature prevailing at the bottom of the tower. The temperature at the top of the side stream tower 3|) is 145 F. In tower 33, the same type of temperature, gradient is shown as in the concentrating section of the tower 20.

The isoprene-acetone-water azeotropic mixture removed as distillate from the tower through lline `3l is conducted to the water scrubbing equipment 4l). Equip-ment 40 may be any suitable type of a liquid-liquid contacting device, in which countercurrent `iiow of y,the Lmaterials may be maintained.' In thepresentcase, the tower is packed with` Raschig ringsfand suflcient pressure is maintained on the tower to prevent vaporization of any ofthe materials passing through it. The stream removed from the system through line 3l' is admitted to the bottom of the tower 4U Y irough adistributing device and then allowed toiv now upward countercurrent to a stream of l water admitted to thetowerjthrough line 44,

In passing through the tower, the acetone is 'removed and the hydrocarbon `is thus completely freed `of the solvent employed in the extractive distillation. The quantity of water employed is determined by the partition of the solvent between the hydrocarbon and the water and the :efciency of `the extracting equipment 4D.- For the specific composition taken, a Volume ratio of water to distillate'is approximately 1. Passing 'overhead from the tower 48 is partially purified isoprene in a high' concentration.` Impurities `present in the streamconsist of piperylene, cyclopentadiene, polymers and any-other water insoluble materials contained in the distillate from `thetower 30. `The water extract removed from the tower through line 48 consists of a dilute solution of acetone in water. This extract is combined with a similar extract from the tower 5l) Tand treated for the recovery of the acetone solvent asto be subsequently described.

The solvent-free diolefln concentrate from the tower is conducted through line` 46 to the iracltionating tower 13. y This tower may be any suitable device for effecting a fractional distillation.

For the speciiic` composition chosen, a tower containing .50 bubbleLcap plates is employed. The

tower 1E! is provided with an overhead vapor line l2, a condenser '13, a reflux drum 14, 1a reflux line 'l5 and a bottoms withdrawal line 16, a reboiler i3 and a reboiler vapor return line 19. The distillate product is withdrawn through line 8| and maintained upon the tower l is between 12 4and 15 lbs, per sq. in. (gauge).

The distillate withdrawn through line l2 con sists of highly puried isoprene containing traces of piperylene and pentenes. The distillate is condensed in equipment T3 and the condensate conducted to the reflux drum 14. -A portion of condensate is returned to the tower 'I0 as reflux through line 15, while the remainder is withdrawn as product through line 8|. It is cooled in equipment Si! and transferred to Astorage through line The bottoms consisting of the piperylene and cyclopentadiene, in addition to any polymer or high boiling constituents, of the feed to the tower and containing some isoprene, is withdrawn through line 1S. A portion of the material is passed to the reboiler 18 where it is vaporized, the

vapors being returned to the tower 'lll through line 'H9 to supply the heat necessary to operate the tower. The remainder of the bottoms are removed through the line 11 and recycled to the feed tower l0 for the'recovery of any isoprene content.

Bynincreasing the number-of-p-lates in tower 10 or increasing the reflux ratio, -essentialh7 all of `the isoprene may be taken overhead as purified product. It has been found, however, more elli- Acient tojreject a small amountof isoprene to the bottoms and lrecycle same `for recovery as described. -A reflux ratio vof l2 is vmaintained for thedesired separation in the tower 10. The distillate fraction from the tower 20 which Y 50 by meanszof the line 43. The extraction tower I`i) is similar to the 4extraction tower 4D. It is similarly packed with Raschigvrings. Thecom- Dosite solution consisting ofthe Ct oleiins and parailins, in addition to acetone and water, is introduced into the tower 50 near the bottom and passes upward through the tower countercurr ently to a'stream of'water introduced near the top through line 4,5. n

In vpassing through the tower, the solvent is extracted from the hydrocarbon material. Any

Y polymers or other high boiling material in the portion of the .recycled solvent which was combined with the distillate from vtower 2i) is extracted by the hydrocarbons of the distillate material. The hydrocarbons freed of solvent are removed through line 41 at the top of the tower 5 Yand discarded. -The water extract consisting of a dilute solution of acetone in water is withdrawn through line 49 at the ybottom of the tower 50 and after being combined with a similar extract -from the tower 4l) is transferred through line 5l, through heat exchangers 52 and line `6| from the tower E@ consists of relatively pure solvent. The tower is operated at about atmospheric pressure. The distillate vapor passes through line .52 tothe condenser 63 and the condensate passed to reflux drum 64. A portion of the condensate is returned to the tower 60 through line 65, while the remainder is withdrawn and transferred through line 6H to line 39 where it combines with the solvent admitted to the tower 28. VHeat is supplied'to the tower 5% by means of .a steam line G8. through which open steam is `injected directly into the bottom of the tower. The bottoms comprising the major part of the water content of the feed and the condensed steam is withdrawn through line 66 to the heat exchanger 52. Thefhea't exchanger 52 is utilized to heat the feed Vto the tower Si?. Thus, the sensible heat of the bottoms is utilized.

The reflux returned to the tower 6B through line 65 is controlled'at-such a rate that the water content of the distillate passing through line 62 and consequently the Vproduct returned to the tower 30' through lines 59 andr' is the same as the recycled solvent Vin toweril. Byy proper adjustment of the reflux ratio in tower 69, the water content of the product may be varied to correct the variations in the Water `content of the solvent in the tower 24. Normally a reilux ratio of about 0.5 is maintained .in the tower 60.

With the foregoing disclosure and illustration ofthe invention .itwill be obvious to those skilled in'the art that various combinations and variations of the` applications of the principle described and illustrated can be made without departing from the inventive concept and that the finvention in, the broad aspect comprises the unique combination of fractionally' distilling and I extractively distilling suitable. feed `stock materials for the separation of individual hydrocarbon compounds. u

What is claimed is:

l. A process for separating isoprene from a refinery cracked stock containing isoprene which 

